An antenna unit installed on a roof of a vehicle or the like has a structure in which a base plate is fixed to an exterior plate and an outer cover appearing externally is mounted on the base plate so that an antenna member is contained inside. In this antenna unit, the outer cover is uniquely shaped and colored to improve not only functionality but also design quality.
As an antenna unit of this type, one in a double case structure configured of a base plate, an inner cover, and an outer cover has been known. In this case, the base plate is fixed to a vehicle main body, and the outer cover is mounted so as to cover the base plate and the inner cover. For example, in the case of the double case structure, to allow the outer cover to be mountable after the inner plate is attached to the base plate, an engaging part is provided which fixes the outer cover and the base plate, or the outer cover and the inner cover.
In the engaging part to mount the outer cover on the base plate or the inner cover, in many cases, a structure is adopted in which a fixing piece having a locking claw is provided to the outer cover and an engaging part which engages with the locking claw is provided on a base plate or inner cover side (refer to Japanese Patent Application Laid-Open No. 2006-345083, U.S. Pat. No. 7,429,958, and Japanese Patent Application Laid-Open No. 2012-085044).